Clarify JOREK neighbour sentinels

[krystophny]

Problem

The JOREK restart reader says -1 is the boundary marker for neighbours. Model-303 restarts can contain two nonpositive sentinels: -1 at collapsed-axis sides and 0 at physical-boundary sides. The reader already preserves both values, but its public contract and synthetic round-trip fixture cover only -1.

This PR documents the stored-value contract and adds 0 to the existing synthetic round trip. It changes no read path.

Numerical and interface invariants

• Neighbor values remain unmodified after HDF5 input.
• Array shape, Fortran indexing, restart validation, ABI, memory layout, and HDF5 schema are unchanged.
• JOREK geometry, field evaluation, locator behavior, units, and boundary conditions are unchanged.
• The fixture now proves that positive neighbors, zero physical-boundary sentinels, and negative collapsed-axis sentinels all round-trip.

Verification

Test fails on main

The documented nonpositive-sentinel contract and zero fixture are absent:

$ rg 'nonpositive boundary' src/field/jorek_restart.f90
exit=1

Test passes after fix

$ fo
Static: OK (227 modules, 227 changed, 227 affected)
Build: OK
Tests: OK
Lint: OK
All stages passed (.4s)